1. Field of the Invention
The present invention relates to a cathode ray tube of a monitor, and more particularly to a circuit for controlling a horizontal focus signal in a monitor wherein an amplification factor of a horizontal focus signal is controlled in accordance with a DC power supply that is varied by a varied horizontal frequency for improving picture quality.
2. Description of the Prior Art
A cathode ray tube of a general monitor focuses and accelerates thermoelectrons emitted from an electron gun, and leads the focused and accelerated thermoelectrons to collide onto a phosphor layer via apertures perforating through a shadow mask to form pixels. At this time, the thermoelectrons emitted from the electron gun are deflected in the horizontal and vertical direction associative with a current of sawtooth waveform that flows through vertical and horizontal deflection coils.
However, since the phosphor layer of a general monitor has a dished plane, resolution is degraded at the periphery of a picture when the electron beam is deflected by centering about the center of the monitor. In order to improve the resolution at the periphery of the picture, a parabola-shaped focus signal is required for permitting the electron beam to accurately scan the phosphor layer. At this time, the parabola-shaped focus signal is classified into a static focus signal of which peak value is fixed and a dynamic focus signal of which peak value is varied in accordance with a frequency of a horizontal sync signal.
A circuit for generating a dynamic focus signal in a general monitor is illustrated in FIG. 1. Referring to FIG. 1, a reference numeral 11 denotes an oscillating section which is oscillated in accordance with frequencies of externally-supplied horizontal sync signal Hs and vertical sync signal Vs to generate a horizontal deflection signal and a vertical deflection signal; 12, a horizontal output section for controlling the output of the horizontal deflection signal in accordance with a scanning interval and flyback interval; and 13, a horizontal deflection yoke for supplying the horizontal deflection signal from horizontal output section 12 to deflect the electron beam in a horizontal direction.
A reference numeral 15 denotes a horizontal focus signal generating section for producing a parabola-shaped focus signal from an output signal of horizontal deflection yoke 13; 16, a horizontal focus signal-amplitude controlling section which is switched in accordance with the horizontal focus signal to regulate an amplitude of the horizontal focus signal; and 17, a horizontal focus signal amplifying section for amplifying an output of horizontal focus signal-amplitude controlling section 16.
A reference numeral 21 denotes a vertical output section for controlling an output of the vertical deflection signal in accordance with the scanning interval and flyback interval; and 22, a vertical deflection yoke for supplying the vertical deflection signal from vertical output section 21 to deflect the electron beam in the vertical direction.
A reference numeral 23 denotes a vertical focus signal generating section for generating a parabola-shaped vertical focus signal from an output signal of vertical deflection yoke 22; and 24, a vertical focus signal amplifying section for amplifying the vertical focus signal of vertical focus signal generating section 23.
A reference numeral 31 denotes a mixing section for mixing the horizontal focus signal with vertical focus signal to provide a dynamic focus signal; and 32, a power supplying section for shaping an externally-supplied AC power supply AC to provide a DC power supply B+, and overlapping DC power supply B+ with the dynamic focus signal. Also, a reference numeral 33 denotes a high voltage generating section for providing a high voltage in accordance with an overlap signal, and supplying the generated high voltage to a grid terminal G4 of the cathode ray tube.
A reference numeral 34 denotes a DC power supply controlling section for generating a pulsewidth modulation (PWM) signal corresponding to DC power supply B+ detected by an auxiliary coil Lf of high voltage generating section 33, generating a voltage in accordance with the PWM signal and supplying the generated voltage to high voltage generating section 33 as DC power supply B+.
Here, horizontal focus signal generating section 15, horizontal focus signal-amplitude controlling section 16 and horizontal focus signal amplifying section 17 are as shown in FIG. 2. In FIG. 2, horizontal focus signal generating section 15 is formed by a resistor 111 and a capacitor 112 serially connected to each other for integrating the output signal of horizontal deflection yoke 13, an inductor 113 for storing a discharge energy of resistor 111 and capacitor 112, and a resistor 114 and a capacitor 115 serially connected to each other for shaping an output signal of capacitor 112.
Horizontal focus signal-amplitude controlling section 16 includes a resistor 121 and a capacitor 122 serially connected to each other for shaping the horizontal focus signal of horizontal focus signal generating section 15. Also included as parts are resistors 123 and 124 serially connected to each other for diving an output signal of capacitor 122, and a Zenor diode 125 connected to an output side of capacitor 122 for switching in the on state when the output signal of capacitor 122 is a voltage of exceeding a preset value. Also, a zener diode 126 is connected to the output side of resistor 123 for switching in the on state when an output signal of resistor 123 exceeds a preset value. Resistors 127 and 128 respectively bias output voltages of zener diodes 125 and 126, and transistors 129 and 130 are respectively switched in accordance with output voltages of resistors 127 and 128. A capacitor 131 and a resistor 132 are serially connected to each other for shaping an output signal of resistor 114. Further to these, resistors 133, 134 and 135 respectively connected between the output side of resistor 132, a collector side and an emitter sides of transistors 129 and 130 control output current of capacitor 131 and resistor 132 in accordance with the switching state of transistors 129 and 130 to regulate the amplitude of an output signal of resistor 132.
Horizontal focus signal amplifying section 17 has a capacitor 140 for eliminating a DC component of the horizontal focus signal of which amplitude is controlled by resistors 132, 133, 134 and 135, and a transistor 141 for amplifying an output signal of capacitor 140. Also, a plurality of resistors 142, 143 and 144 are respectively connected to a collector side of the transistor 141, between an emitter side and a ground of the transistor 141, and between the collector side and the base side of the transistor 141. for determining an amplification factor of an output signal of capacitor 140. A transistor 145 for buffering the output signal of transistor 141, and a resistor 146 connected to an emitter side of transistor 145 for determining the output current of transistor 145 are provided.
On the other hand, vertical output section 21, vertical deflection yoke 22, vertical focus signal generating section 23 and vertical focus signal amplifying section 24 are operated identically to horizontal output section 12, horizontal deflection yoke 13, horizontal focus signal generating section 15 and horizontal focus signal amplifying section 17. Thus, a detailed description of the operation will be deleted.
DC power supply controlling section 34 includes a voltage detector 151 having a diode and a capacitor for rectifying and shaping an output signal of auxiliary coil Lf of high voltage generating section 33 which detects DC power supply B+ to detect an output voltage of high voltage generating section 33. In addition to voltage detector 151, a pulsewidth modulator (PWM) 152 generates a PWM signal for producing DC power supply B+ in accordance with an output voltage of voltage detector 151, and a voltage controller 153 produces DC power supply B+ in accordance with the PWM signal to supply it to high voltage generating section 33.
Here, PWM 152 is provided to produce the PWM signal for heightening DC power supply B+ from power supplying section 32 when the horizontal frequency is raised to thus lower the output voltage of voltage detector 151.
In the general circuit for generating the dynamic focus signal formed as above, horizontal sync signal Hs and vertical sync signal Vs are externally supplied to oscillating section 11. Then, oscillating section 11 is oscillated in accordance with the frequencies of horizontal sync signal Hs and vertical sync signal Vs to generate the horizontal deflection signal and vertical deflection signal. The generated horizontal deflection signal and vertical deflection signal are respectively supplied to horizontal output section 12 and vertical output section 21.
Horizontal output section 12 controls the output of horizontal deflection signal in accordance with the scanning interval and flyback interval, and the horizontal deflection signal of horizontal output section 12 is supplied to horizontal deflection yoke 13. Then, horizontal deflection yoke 13 deflects the electron beam in the horizontal direction.
The output signal of horizontal deflection yoke 13 is supplied to horizontal focus signal generating section 15 which in turn generates the parabola-shaped horizontal focus signal. The horizontal focus signal is supplied to horizontal focus signal-amplitude controlling section 16.
Horizontal focus signal-amplitude controlling section 16 reduces the amplitude of the horizontal focus signal when the amplitude of the horizontal focus signal exceeds the preset value. The output signal of horizontal focus signal-amplitude controlling section 16 is supplied to horizontal focus signal amplifying section 17.
Thereafter, horizontal focus signal amplifying section 17 amplifies the horizontal focus signal to supply the amplified signal to mixing section 31.
Meanwhile, vertical output section 21 controls the output of the supplied vertical deflection signal in accordance with the scanning interval and flyback interval, and the output signal of vertical output section 21 is supplied to vertical deflection yoke 22 which then deflects the electron beam in the vertical direction.
The output signal of vertical deflection yoke 22 is supplied to vertical focus signal generating section 23. Then, vertical focus signal generating section 23 produces the parabola-shaped vertical focus signal in accordance with the vertical deflection signal of sawtooth waveform from vertical deflection yoke 22. The vertical focus signal is supplied to vertical focus signal amplifying section 24 which then amplifies the vertical focus signal in accordance with the preset value.
The amplified horizontal focus signal and vertical focus signal are supplied to mixing section 31. Then, mixing section 31 mixes the amplified horizontal focus signal and vertical focus signal to produce the dynamic focus signal which is in turn supplied to high voltage generating section 33.
On the other hand, externally-supplied AC power supply AC is provided to power supplying section 32 which shapes AC power supply AC to provide DC power supply B+. Thus, DC power supply B+ overlaps with the dynamic focus signal, and the overlap signal is supplied to high voltage generating section 33. In accordance with the overlap signal, high voltage generating section 33 provides the high voltage, and the generated high voltage is supplied to grid terminal G4 of the cathode ray tube.
DC power supply B+ detected by auxiliary coil Lf of high voltage generating section 33 is supplied to DC power supply controlling section 34. Successively, DC power supply controlling section 34 detects the output voltage of high voltage generating section 33 operated in accordance with DC power supply B+ supplied, generates the PWM signal for generating DC power supply B+ corresponding to a mode in accordance with the detected DC voltage, and produces the DC power supply corresponding to the mode in accordance with the generated PWM signal to supply it to high voltage generating section 33.
The process of generating the parabola-shaped focus signal will be described with reference to FIG. 2.
The output signal of horizontal deflection yoke 13 is supplied to resistor 111 and capacitor 112 of horizontal focus signal generating section 15. Resistor 111 and capacitor 112 integrate the output signal of horizontal deflection yoke 13, and the output signal of resistor 111 and capacitor 112 is supplied to inductor 113. In turn, inductor 113 stores the energy discharged from resistor 111 and capacitor 112.
The output signal of resistor 111 and capacitor 112 is supplied to resistor 114 and capacitor 115, and resistor 114 and capacitor 115 shape the output signal of resistor 111 and capacitor 112.
The shaping signal of resistor 114 and capacitor 115 is sequentially supplied to resistor 121 and capacitor 122 of horizontal focus signal generating section 16. Resistor 121 and capacitor 122 shape the horizontal focus signal. Then, the resultant shaping signal is supplied to resistors 123 and 124 to be divided, and the resultant shaping signal and the divided signal are respectively supplied to zener diodes 125 and 126.
Zener diodes 125 and 126 are switched in a manner that zener diode 126 is switched in the turn-on state when the divided signal is more than a preset value, e.g., approximately 16 volts, of zener diode 125; whereas, zener diode 125 is switched in the turn-on state when the divided signal is more than a preset value, e.g., approximately 27 volts, of zener diode 126.
The output voltages of zener diodes 125 and 126 are respectively supplied to transistors 129 and 130 via resistors 127 and 128. Transistors 129 and 130 are switched in accordance with the switching state of zener diodes 125 and 126.
In other words, when the divided signal is more than the preset value of zener diode 125, transistors 129 and 130 are switched in the turn-on state. If the divided signal is more than the preset value of zener diode 126, transistor 130 is switched in the turn-on state.
Meanwhile, the horizontal focus signal of horizontal focus signal generating section 15 is sequentially supplied to capacitor 131 and resistor 132, and capacitor 131 and resistor 132 shape the horizontal focus signal.
Then, the amplitude of the shaped horizontal focus signal and the divided signal are varied in accordance with the switching state of transistors 129 and 130.
That is, when the amplitude of the shaped horizontal focus signal is smaller than the preset values of zener diodes 125 and 126, transistors 129 and 130 are switched in the turn-off state. The horizontal focus signal shaped in accordance with the turn-off state of transistors 129 and 130 is supplied to resistors 132, 133, 134 and 135, and the amplitude of the shaped horizontal focus signal is controlled in accordance with the divided values of resistors 132, 133, 134 and 135.
However, if the amplitude of the shaped horizontal focus signal exceeds the preset value of zener diode 126 to switch transistor 130 in the turn-on state, the shaped horizontal focus signal is supplied to resistors 132, 133 and 134. Then, resistors 132, 133 and 134 divide the shaped horizontal focus signal. In accordance with the divided resistance values, the amplitude of the shaped horizontal focus signal is controlled.
On the other hand, if the amplitude of the shaped horizontal focus signal is more than the preset value of zener diode 125 to switch transistors 129 and 130 in the turn-on state, the shaped horizontal focus signal is supplied to resistors 132 and 133. After this, resistors 132 and 133 divide the shaped horizontal focus signal. Also, in accordance with the divided resistance values, the amplitude of the shaped horizontal focus signal is controlled.
The horizontal focus signal of which amplitude is controlled as described above is supplied to horizontal focus signal amplifying section 17.
The horizontal focus signal with its amplitude controlled is supplied to capacitor 140 of horizontal focus signal amplifying section 17. Capacitor 140 eliminates the DC component of the output signal of horizontal focus signal-amplitude controlling section 16. Then, the output signal of capacitor 140 is supplied to transistor 141 which amplifies the output signal of capacitor 140. At this time, the amplification factor of transistor 141 is determined in accordance with the resistance values of plurality of resistors 142, 143 and 144.
The output signal of transistor 141 is supplied to transistor 145 which buffers the output signal of transistor 141. At this time, the output current of transistor 141 is determined by the resistance value of resistor 146.
Here, vertical output section 21, vertical deflection yoke 22, vertical focus signal generating section 23 and vertical focus signal amplifying section 24 are operated identically to horizontal output section 12, horizontal deflection yoke 13, horizontal focus signal generating section 15 and horizontal focus signal amplifying section 17. Thus, a detailed description of them will be deleted.
Additionally, a process of regulating DC power supply B+ detected by auxiliary coil Lf of high voltage generating section 33 will be described in more detail with reference to FIG. 3.
The output voltage of auxiliary coil Lf of high voltage generating section 33 is sequentially supplied to the diode and capacitor of voltage detector 131, and the diode and capacitor rectify and shape the output signal of auxiliary coil Lf to provide the DC voltage.
The DC voltage of voltage detector 151 is supplied to PWM 152. Thereafter, PWM 152 produces the PWM signal for generating the DC power supply corresponding to the mode in accordance with the output voltage of voltage detector 151, and the PWM signal is supplied to voltage controller 153. Then, voltage controller 153 generates DC power supply B+ corresponding to the mode in accordance with the PWM signal and supplies the generated DC power supply to high voltage generating section 33.
In the conventional dynamic focus signal generating circuit as described above, the amplification of the horizontal focus signal is varied in accordance with the amplitude of the horizontal focus signal. For this reason, the the horizontal and vertical focus signals are difficult to be finely amplified when the frequency of the horizontal sync signal is minutely varied, which results in degrading the resolution of the monitor.